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Pilot's Handbook of Aeronautical Knowledge
Aerodynamics of Flight
Forces Acting on the Aircraft

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




• Drag—a rearward, retarding force caused by
disruption of airflow by the wing, rotor, fuselage, and
other protruding objects. Drag opposes thrust, and acts
rearward parallel to the relative wind.

• Weight—the combined load of the aircraft itself, the
crew, the fuel, and the cargo or baggage. Weight pulls
the aircraft downward because of the force of gravity.
It opposes lift, and acts vertically downward through
the aircraft's center of gravity (CG).

• Lift—opposes the downward force of weight, is
produced by the dynamic effect of the air acting on
the airfoil, and acts perpendicular to the flightpath
through the center of lift.

In steady flight, the sum of these opposing forces is always
zero. There can be no unbalanced forces in steady, straight
flight based upon Newton's Third Law, which states that for
every action or force there is an equal, but opposite, reaction
or force. This is true whether flying level or when climbing
or descending.

It does not mean the four forces are equal. It means the
opposing forces are equal to, and thereby cancel, the effects
of each other. In Figure 4-1 the force vectors of thrust,
drag, lift, and weight appear to be equal in value. The usual
explanation states (without stipulating that thrust and drag
do not equal weight and lift) that thrust equals drag and lift
equals weight. Although basically true, this statement can
be misleading. It should be understood that in straight, level,
unaccelerated flight, it is true that the opposing lift/weight
forces are equal. They are also greater than the opposing
forces of thrust/drag that are equal only to each other.
Therefore, in steady flight:

• The sum of all upward forces (not just lift) equals the
sum of all downward forces (not just weight).

• The sum of all forward forces (not just thrust) equals
the sum of all backward forces (not just drag).

Relationship of forces acting on an airplane.
Figure 4-1. Relationship of forces acting on an airplane.

Force vectors during a stabilized climb.
Figure 4-2. Force vectors during a stabilized climb.

In glides, a portion of the weight vector is directed forward,
and, therefore, acts as thrust. In other words, any time
the flightpath of the aircraft is not horizontal, lift, weight,
thrust, and drag vectors must each be broken down into two

Discussions of the preceding concepts are frequently omitted
in aeronautical texts/handbooks/manuals. The reason is not
that they are inconsequential, but because the main ideas
with respect to the aerodynamic forces acting upon an
airplane in flight can be presented in their most essential
elements without being involved in the technicalities of the
aerodynamicist. In point of fact, considering only level flight,
and normal climbs and glides in a steady state, it is still true
that lift provided by the wing or rotor is the primary upward
force, and weight is the primary downward force.

By using the aerodynamic forces of thrust, drag, lift, and
weight, pilots can fly a controlled, safe flight A more detailed
discussion of these forces follows.

For an aircraft to move, thrust must be exerted and be greater
than drag. The aircraft will continue to move and gain
speed until thrust and drag are equal. In order to maintain a
constant airspeed, thrust and drag must remain equal, just as
lift and weight must be equal to maintain a constant altitude.
If in level flight, the engine power is reduced, the thrust is
lessened, and the aircraft slows down. As long as the thrust is less than the drag, the aircraft continues to decelerate until
its airspeed is insuficient to support it in the air.